Many electrical and electronic devices require a power supply for converting electrical voltage and current at given levels to levels usable by the device. One of the most common types of power supply is the so called wall mount transformer, which comprises a transformer circuit for converting an alternating current (AC) voltage at a commercial level, such as 120 volts AC, to a direct current (DC) level usable by the device. Indeed, such supplies are sold by the millions worldwide for devices that can work efficiently at a single output DC level. Typically these types of supplies use a simple step down transformer, appropriately designed, with a rectifier circuit on the output. Other types of devices, such as, for example, battery chargers and mobile computer power supplies, require output levels that may change during use, or that may be of such magnitude that the step down transformer approach results in a power supply that is too bulky for mobile applications.
In recent years there has been a drive towards mobility for electronic devices. This is especially true for communications devices such as cellular radio-telephones, and computing equipment such as laptop computers. In many instances the use of such equipment is desirable in automobiles, and as such, there are numerous accessories available that permit the use of a vehicular cigarette lighter socket to power these types of devices. However, in order to use both an AC source, and a DC source, the user of the device typically must purchase separate power supplies; one for use with an AC source, and one for use with a DC source. It would therefore be desirable to have a power supply that can operate from both AC and DC sources, thus relieving the burden of having to purchase and keep two different power supplies. Although a dual range power supply would be fairly simple to realize, to date there has not been a strong presence of such a power supply in the marketplace.
One reason that may account for the virtual absence of a dual range power supply for mobile equipment is a safety concern. Connectors for AC and DC power inputs are typically dissimilar, and therefore two connectors are used, one for each type of input. However, when operating the power supply from an AC source, it may be possible through some sort of equipment malfunction that the AC power becomes evident at a DC connector, and may pose a shock hazard. Furthermore, a hazard potential may exist if the user were able to provide both types of power simultaneously. That is, connecting to AC and a DC sources at the same time.
Therefore there exists a need for a dual range power supply operable from either an AC source or a DC source in a manner such that the simultaneous interconnection of the two different types of input is barred. In addition, the device must accommodate two very different types of electrical receptacles in order to be successfully utilized in both AC line powered and DC vehicular powered applications. Such a power supply would offer ease of use, mobility, and eliminate the safety concerns described above.